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Flimsy rocks allowed Earth’s plates to start moving

FRAGILE things can be useful. It seems that the shifting plates of rock that make up Earth’s crust can only move because the crust is partly made of damaged rocks.

Uniquely in the solar system, Earth’s crust is divided into several tectonic plates that move over the aeons. Often, one plate gets subducted under another and sinks into the mantle beneath, while elsewhere, new rocks rise to the surface. If the plates did not recycle like this, the surface might not be so rich in the chemicals vital for life.

But it was unclear how the plates started moving. The first subduction was probably about 4 billion years ago, but it took another billion years for all the plates to move.

To explain this delay, David Bercovici of Yale University and Yanick Ricard of the University of Lyon in France modelled rocks in the highly viscous mantle. There, strong currents create weak zones in the rocks.

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Over time, more of these damaged rocks were thrust back up to form new sections of plate. Because these new plates were partly made of fragile rocks, their edges were more breakable, making it easier for one to slide under another (Nature, doi.org/r7m).

Bercovici says that may explain the delay&colon; it took a billion years to make the plates fragile enough that subduction could get going in earnest.

“Their model makes intuitive sense,” says Catherine McCammon of the University of Bayreuth in Germany.